Fluoropolymer coating composition

ABSTRACT

Aqueous fluoropolymer formulations containing in finely divided form from 15 to 75% by weight of a fluoropolymer which preferably has at least 40% by weight of tetrafluoroethylene units, from 0.5 to 10% by weight of an organic compound which has at least two, preferably 2-4, isocyanate groups and, optionally, up to 20% by weight of auxiliary or additive substances are prepared and used for coating yarn, in particular sewing yarn, and textile sheet materials.

This is a division of my application Ser. No. 07/307,516, filed Feb. 2,1989, now U.S. Pat. No. 5,242,962, issued Sep. 07, 1993.

The present invention relates to an aqueous fluoropolymer formulationand to the preparation thereof, this fluoropolymer formulation making itpossible to produce strongly adherent fluoropolymer coatings on textilematerials made of organic synthetic fibers.

European Patent Application 0,224,262 discloses laminating textile sheetmaterials on both sides with a polyvinyl fluoride coating. In this knownprocess, a solution of the polymer in an organic solvent is cast on aconveyor belt into a thin layer which made to gel by heating. The gellayer is then pressed onto the surface of the textile material to belaminated. In this process, the fluoropolymer coating is fixed to thetextile material only at the surface and virtually no impregnation ofthe individual filaments with the fluoropolymer takes place.

German Offenlegungsschrift 3,301,270 discloses a process for sheathingfiber or filament yarns with a fluorine-containing polymer. In thisresource-intensive process, the filament to be sheathed is pulledthrough a central hole in an annular spinneret while at the same time atube of fluoropolymer is extruded from the annular spinneret. Theextruded tube therefore surrounds the filament drawn out of the centralhole as a loose-fitting sheath. No firm bonding takes place between thefluoropolymer tube and the filament sheathed therewith.

It is also known for the manufacture of awnings, airhouses, flexiblecontainers and similar products to coat textile sheet structures,usually woven fabrics, preferably those made of synthetic organic fibersor filaments, with polymer materials, usually with polyvinyl chloride(PVC). This coating is effected by impregnating the textile materials insuspensions of polyvinyl chloride in organic liquids. In the course ofthis coating, even the individual filaments of the textile materialbecome enveloped by the polyvinyl chloride coating. To obtain sufficientadhesion between the polyvinyl chloride coating and the syntheticfibers, the coating is carried out in two stages. First a basecoat isapplied comprising a mixture of a PVC paste or suspension with anadhesion promotor; this is followed by the application of a topcoatcomprising a pure PVC formulation. The adhesion promotors suitable forthis purpose are known. It is usual to use two-component adhesivescomprising an organic substance having a plurality of hydroxyl groups,preferably a hydroxyl-containing polyester, and an organic substancehaving a plurality of isocyanate groups.

It is also already known to coat materials such as threads or sheetlikestructures made of organic synthetic fibers with fluoropolymers in orderto confer particularly advantageous properties, for example a lowco-efficient of friction, a high chemical resistance and asoil-repellent effect, on their surfaces. To this end, the syntheticfiber materials are impregnated or coated with commercially availableaqueous dispersions of fluoropolymers and the resulting polymericcoating is fixed by means of a heat treatment.

However, in order to make composite materials based onfluoropolymer-treated synthetic fibers suitable for a wide range ofapplications, for example the manufacture of membranes for textileconstruction, flexible containers, conveyor belts, fabric tubes and thelike, it is absolutely necessary that the fluoropolymer should showadequate adhesive strength to the synthetic fiber. Adhesive strength ishere to be understood as meaning the resistance to separation of basematerial and coating for a 5 cm wide strip as determined in line withGerman Standard Specification DIN 53 530. Adequate performancecapability of the composite is ensured when, depending on the intendeduse, adhesion values of from 10 to 15 daN/5 cm are obtained.

However, the production of strongly adherent fluoropolymer coatings ofsynthetic fiber materials presents even greater difficulties than theproduction of polyvinyl chloride coatings. This is because it is foundthat fluoropolymers are far more inert with respect to synthetic fibers,for example polyester fibers, polyamide fibers or aramid fibers, thanpolyvinyl chloride; that is, they show great reluctance to enterpermanent physical or chemical bonds with synthetic fiber surfaces.Moreover, fluoropolymers which, on the basis of the physical data, mightbe thought suitable for use as coating agents for synthetic fibers arein general commercially available in the form of aqueous dispersions orpastes. It is therefore not possible using these known fluoropolymerdispersions or pastes to produce coatings on synthetic fiber materialsshowing adequate adhesive strength for all the abovementioned industrialuses.

Nor is it possible to obtain a significant improvement in the adhesionof fluoropolymer coatings by using the one-or two-component adhesionpromotors used successfully in the production of PVC coatings.

It has now been found, surprisingly, that it is possible to produce verystrongly adherent fluoropolymer coatings on synthetic fibers by applyingat least as the first impregnation or as the first coat (basecoat) afluoropolymer formulation which, in place of conventional adhesionpromotors, merely contains an organic compound having a plurality ofisocyanate groups.

The present invention thus provides an aqueous fluoropolymer formulationcontaining in finely divided form from 15 to 75% by weight, preferablyfrom 25 to 60% by weight, of a fluoropolymer, from 0.5 to 10, preferablyfrom 1 to 5% by weight of an organic compound which has at least two,preferably 2 to 4, isocyanate groups and, optionally, up to 20% byweight, preferably up to 10% by weight, of auxiliary or additivesubstances.

The fluoropolymers of the formulation according to the invention containto an extent of at least 40% tetrafluoroethylene units and to an extentof up to 60% units derived from other unsaturated fluorine-containingmonomers copolymerizable with tetrafluoroethylene. Advantageously, theamounts and identities of the fluorine-containing monomers copolymerizedwith the tetrafluoroethylene are chosen in such a way that the copolymerhas the desired mechanical properties, such as strength, flexibility,elasticity, low slip friction and the like, and also good chemicalresistance. More particularly, the composition should be chosen in sucha way that the fluoropolymer is readily dispersible in water and alsogives dispersions of good long term stability. Preference here is givento those fluoropolymers whose melting points are below the melting pointof the synthetic fibers to be finished with the formulation. Moreparticularly, fluoropolymers of advantage here are those whose meltingpoints are from 5° to 50°, prefereably from 20° to 40° C., below themelting point of the synthetic fibers to be finished.

High suitability within the meaning of the above specifications ispossessed by tetrafluoroethylene copolymers containing to an extent ofup to 60% units derived from hexafluoropropylene and vinylidenefluoride. A fluoropolymer particularly suitable for preparing thefluoropolymer formulation according to the invention contains from 40 to60% by weight of tetrafluoroethylene units, from 10 to 30% by weight ofhexafluoropropylene units and from 20 to 40% by weight of vinylidenefluoride units.

The preparation of such fluoropolymers is known, and numerousrepresentatives of this class of compounds are commercially available.Particularly high suitability for the preparation of the fluoropolymerformulation according to the invention and for producing stronglyadherent fluoropolymer coatings on synthetic fibers is possessed by therange of ®Hostaflon TFB products available from Hoechst AG.

Organic compounds which have a plurality of isocyanate groups and whichcan be incorporated as adhesion promotors into the fluoropolymerformulations according to the invention are known in large numbers, andnumerous representatives of this class of compounds are commerciallyavailable. A number of industrially important tri- and polyisocyanateswhich are also suitable for the use according to the invention may befound for example in Ullmann's Enzyklopadie der technischen Chemie, 4thedition, Volume 13, page 355, Volume 15, page 634 and Volume 19, pages303 and 304. It is of course also possible to use other di- andpolyisocyanates which are dispersible and sufficiently stable in aqueoussystems in the fluoropolymer formulations according to the invention.Preferred di- and polyisocyanates are for example the isomeric2,4-diisocyanatotoluenes and mixtures thereof,1,5-diisocyanatonaphthalene, diisocyanatodiphenylmethane and itstechnical-grade isomer mixtures, dimerized and trimerizeddiisocyanatotoluene, and adducts of diisocyanatotoluene withtrimethylolpropane and tris[isocyanatohexyl]biuret. Particularpreference for use in the fluoropolymer formulations according to theinvention is given to the aforementioned derivatives ofdiisocyanatotoluene, in particular its dimerization product, which isavailable from Bayer AG under the name ®Desmodur TT.

As additives there may be present in the fluoropolymer formulationsaccording to the invention various auxiliary and additive substances.These include in particular surface-active agents (surfactants) whichmay serve as wetting agents in the preparation of formulations and intheir application and dispersants which facilitate and/or make possiblethe dispersion of fluoropolymers and cyanato-containing compounds in theaqueous phase and increase dispersion stability. Again suchsurface-active agents are described in the literature in large numbers.

We cite for example Ullmann's Enzyklopadie der technischen Chemie, 4thedition, Volume 22, pages 455 et seq., in particular pages 466 and 488and also Volume 23, page 120. It is possible to use for example anionic,cationic or amphoteric surfactants, but preferably nonionic surfactantsderived from ethoxylates, terminally blocked ethoxylates, fatty acidesters and polyhydroxy compounds. The possibilities also includesilicone-based surfactants or fluorosurfactants. If a novelfluoropolymer formulation having a relatively low fluoropolymerconcentration and a correspondingly low viscosity is to be used, it canbe advantageous also to add as an auxiliary and additive substance athickening or antimigration agent, for example agents from the group ofthe carboxymethylated polysaccharides, xanthans or alginates, of thestarch or cellulose derivatives, of the acrylate copolymers or ofpolyvinyl alcohol. Even inorganic thickening agents such as pyrogenicsilica or bentonites may be incorporated into the fluoropolymerformulation according to the invention in low concentration.

For specific applications, in particular for the topcoat, it is alsopossible to use mixtures of formulations according to the invention withblend components specific to the intended use. Such blend components arefor example pigments, fillers, flame-retardant agents and modifyingagents which are capable of modifying the properties of thefluoropolymer coating, for example plasticizers, lubricants or agentswhich modify the surface properties. These can be incorporated into theformulations according to the invention in a total amount of up to 25%by weight, preferably up to 15% by weight. However, it is advantageousnot to use this second group of additives in formulations according tothe invention which are to be used in the first coat (the basecoat).

It is of course advantageous and particularly preferable to choose theamount of the auxiliary or additive substances in the fluoropolymerformulations according to the invention in such a way that the effectintended with these additives is obtained to an optimum degree.

The fluoropolymer formulation according to the invention is prepared ina conventional manner by homogenizing, if 100 parts by weight of theformulation are to be prepared, from 15 to 75 parts by weight offluoropolymer, from 0.5 to 10 parts by weight of the organic compoundhaving at least two isocyanate groups and, optionally, up to 20 parts byweight of further auxiliary or additive substances in finely dividedform with sufficient water to make up to 100 parts by weight, in asuitable mixing apparatus. If the fluoropolymers used here are solid,they must first be converted into a very finely divided form and then byvigorous stirring or kneading into a uniform dispersion or paste in therequisite amount of water, if necessary with the addition of asufficient amount of dispersing auxiliaries and/or wetting agents. Atthe same time or thereafter the requisite amount of the selected organiccompound containing a plurality of cyanate groups is added.

The mixing operation is advantageously carried out at room temperatureor at an only moderately elevated temperature. Further additive orauxiliary substances may be added to the blend batches from the start,or they may be incorporated once a stable fluoropolymer dispersion orpaste has been obtained.

It is advantageous to prepare the fluoropolymer formulations accordingto the invention by incorporating into an aqueous fluoropolymersuspension or fluoropolymer paste which may contain further auxiliary oradditive substances from 0.5 to 10% by weight, based on the final weightof the mixture, a finely divided form of the organic substance whichcontains at least two isocyanate groups, and homogenizing it. Aqueousfluoropolymer suspensions or pastes which are highly suitable forproducing the formulations according to the invention are for examplethe commercially available ®Hostaflon TFB range mentioned above.

Textile materials for the purposes of this invention can be one- ortwo-dimensional; that is, they are threads or else sheetlike structures,for example woven fabrics, knitted fabrics, laid fabrics or nonwovenfabrics of various thicknesses, or even laminates of identical ordifferent sheetlike structures of this type, possibly combined withother raw materials.

The filaments or fibers of the textile materials to be coated can bedyed or undyed, smooth or textured. There is no evidence that commercialtextile dyes migrate out of the synthetic fibers into the fluoropolymercoating.

In principle, the fluoropolymer formulation according to the inventioncan be applied in one operation to the synthetic fiber material to becoated. To produce thicker fluoropolymer coats, the fluoropolymerformulation according to the invention is advantageously applied in aplurality of operations, in which case, depending on the chosencomposition of the formulation and the associated consistency, animpregnation by dipping or padding or alternatively a paste application,for example by knife-coating or roller-coating, may be carried out. Inthis advantageous application of the formulation according to theinvention it is particularly advantageous to carry out the firstapplication, i.e. the production of the basecoat, with a fluoropolymerformulation according to the invention which, aside from th e organiccompound having a plurality of isocyanate groups, contains no or onlylittle other additives, in particular no solid additives, for examplepigments or flame-retardant additives.

It is also possible to prepare fluoropolymer coatings on organicsynthetic fibers by using the fluoropolymer formulation according to theinvention only to prepare the basecoat and to use for the basecoat orbasecoats a normal aqueous fluoropolymer dispersion or paste which isfree of isocyanato-containing organic compounds but which, of course,may contain further additives, for example dispersants, wetting agents,pigments, flame-proofing agents or other filling and auxiliarysubstances.

As stated above, it is also possible to carry out the fluoropolymerfinish using the fluoropolymer formulation according to the invention ina single step, for example by impregnating, this being advantageous inparticular in the application to threads made of synthetic filaments orfibers. In the application of the fluoropolymer formulation according tothe invention, each application by impregnation or coating is in generalfollowed by a heat treatment of the material to dry the application andto fix the freshly applied fluoropolymer to the substrate.

The fluoropolymer coatings applied by the above-specified applicationmethods have a high adhesive strength of at least 10 daN/5 cm, on use ofpreferred formulations according to the invention in general more than20 daN/5 cm. The adhesive strength of the fluoropolymer to the syntheticfiber can be controlled by the amount which is added of the organiccompound containing two or more isocyanate groups. Furthermore, theadhesive strength can be influenced by the temperature and duration ofthe heat treatment carried out following application of the formulationaccording to the invention.

The present invention also provides the textile sheet materials, forexample woven fabrics, knitted fabrics, laid fabrics or nonwoven fabricsmade of synthetic fibers, preferably polyesters, polyamide or aramidfibers, in particular polyester fibers, which are coated with thefluoropolymeric formulation according to the invention. Textile sheetmaterials for the purposes of the present invention also includelaminates of identical or different sheetlike structures, including anymixtures with other raw materials. The coated sheetlike structures arenotable for having, at least in the fluoropolymer layer directlyadjoining the fiber surface, one or more adhesion-promoting constituentsderived from an organic compound having at least two isocyanate groups.Furthermore, the fluoropolymer-coated synthetic fiber textile materialsaccording to the invention are notable for the fact that the coating hasan adhesive strength, as measured in line with German StandardSpecification DIN 53 530, of at least 10 daN/5 cm. Owing to the highmechanical strength properties of polyester, polyamide or aramid fibers,the materials which have been fluoropolymer-coated according to theinvention also have excellent mechanical strength values which very muchwiden their range of possible industrial uses. Moreover, the coatedmaterials have low coefficients of friction, show high chemicalresistance and are soil-repellent.

The present invention further provides the synthetic fiber threadscoated with the fluoropolymer formulation according to the invention.The term threads is here to be understood as meaning monofilaments,multifilament yarns or even staple fiber yarns. Suitable fiber materialhere too comprises in particular polyester, polyamide or aramidsynthetic fibers, preferably polyester fibers. The fluoropolymer-coatedyarns thus obtained have not only the high mechanical strength valuescharacteristic of synthetic fibers but also a particularly high-slip,chemically resistant and soil-repellent surface.

The fluoropolymer formulations according to the invention proveparticularly useful for the bonding of sewing yarn. This is to beunderstood as meaning that the fluoropolymer coating brings about aflexible bond between the individual filaments of the yarn. The bondedsewing yarns thus produced have a particularly high strength andexcellent sewing properties. A particular advantage here is that thebonding action of the extremely inert fluoropolymer coating does notdisappear on dyeing or on application of sewing finishes.

The threads coated according to the invention and the bonded sewing yarnare also notable for the fact that they have, at least in thefluoropolymer layer immediately adjoining to the fiber surface, one ormore adhesion-promoting constituents which are derived from an organiccompound having at least two isocyanate groups and that thefluoropolymer coating has such adhesive strength that it does not becomedetached due to mechanical stress such as is experienced in the furtherprocessing of the threads or in the course of the intended use of thesewing yarn, for example in the course of winding, weaving or knittingor in the course of sewing.

The working examples that follow illustrate the preparation of thefluoropolymer formulation according to the invention and its applicationto yarn and textile sheet materials.

EXAMPLE 1

500 g of a commercially available 50% strength by weight aqueous pasteof a fluorocopolymer of tetrafluoroethylene, hexafluoropropylene andvinyl difluoride (®Hostaflon TFB X 7900 from Hoechst AG) containing 2%by weight of a thickener based on an acrylate polymer and 15 g of afinely pulverulent1,3-bis-(4-methyl-3-isocyanatophenyl)-1,3-diazacyclobutane-2,4-dione(for example Desmodur TT from Bayer AG) are kneaded for 15 minutes in akneader at 20°-25° C. and packaged. This gives a pasty fluoropolymerformulation which is highly suitable for coating textile materials (yarnor textile sheet material) made of synthetic fibers. It contains 48.5%by weight of fluoropolymer, 2.9% by weight of the diisocyanato compoundand 1.9% by weight of thickener. The formulation can also be used togood effect as a primer coating in the coating of textile materials ontowhich firmly adherent fluoropolymer topcoats can be applied by means ofcommercially available fluoropolymer pastes or dispersions.

EXAMPLE 2

10 g of wetting and dispersing agent based on an ethoxylated alkylphenolare dissolved with vigorous stirring in 315 ml of demineralized water at40° C. 175 g of a finely ground fluorocopolymer of 55% by weight oftetrafluoroethylene, 15% by weight of hexafluoropropylene and 30% byweight of vinylidene fluoride are then slowly introduced with stirring.Stirring is continued until the dispersion is completely homogenized,and the dispersion is then packaged. The 35% strength by weight aqueousfluoropolymer formulation obtained is highly suitable for the dipimpregnation and the coating of textile sheet materials and inparticular yarns made of synthetic fibers.

EXAMPLE 3

Coating of woven polyester fabric

A 220 g/m² woven polyester fabric with 9 ends and 9 picks of 1100 dtexper cm is initially provided with the following basecoat:

A fluoropolymer paste according to Example 1 is applied to both sides ofthe fabric, in each case at a rate of 60-80 g/m², by spread coating bymeans of an air knife or a rubber blanket doctor blade.

The material is then sintered at 210° C. for 2 minutes. Topcoating iseffected with a 50% strength ®Hostaflon TFB X 7900 paste (aqueousfluoropolymer paste) which, if desired, may additionally be pigmented orflameproofed (for example by the addition of 5% of TiO₂ and/or 10% ofSb₂ O₃, based on solid fluoropolymer). This formulation is applied byspread coating with a steel roll doctor or rubber blanket doctor bladein a plurality of coats (for example 6 coats on the front, 4 coats onthe back), each coat being followed by an intermediate drying at 210° C.for 2 minutes.

The total amount applied to the front of the fabric is 300 g/m² and tothe back of the fabric 160 g/m².

After the last coat has been applied, the fabric is sintered at 210° C.in the course of a residence time of 10 minutes. Thereafter the surfaceof the front is embossed by means of a calender.

The adhesive strength obtained is 20.7 daN/5 cm.

If this example is repeated exactly as described above with afluoropolymer paste which contains no dicyanato compound but whichotherwise has the composition of the paste prepared in Example 1, anadhesive strength of 9.1 daN/5 cm is found on the front of the coatedfabric and an adhesive strength of 6.9 daN/5 cm on the back.

EXAMPLE 4

Fabric: as in Example 3.

Basecoat: as in Example No. 3, the basecoat being sintered at 210° C. inthe course of a residence time of 10 minutes.

Topcoating: as in Example 3.

Adhesive strength obtained: 30.2 daN/5 cm.

EXAMPLE 5

Fabric: as in Example 3.

Basecoat: as in Example No. 4.

Topcoating: Formulation: Hostaflon TFB X 7100 Melt granules+5% of TiO₂+10% of Sb₂ O₃ +1% of Hoechst Wax OP. Application method: roll meltunit. Amount applied: front 300 g/m² back 160 g/m². Processingconditions: Roll temperatures: 205° C., Preheat roll: 150° C., IRradiator: 90%, Machine speed: 3 m/minute, Rubber roll pressure: 7 bar,Film weight: front 300 g/m², back 160 g/m².

Adhesive strength obtained: 21.5 daN/5 cm.

EXAMPLE 6

Bonding of a sewing thread

A black 266-dtex 64-filament 3-ply polyethylene terephthalate yarn isimpregnated with 15-20% by weight (dry add-on based on weight of fiber)of a formulation as described in Example 2 by dipping.

The impregnated thread is then heat-treated in a hot oven at 220° C. forat least 60 seconds. The sewing thread obtained does not show a changein hue, nor any migration of the dye into the bending. It produces verygood sewing results even under high stress, for example in industrialsewing machines. No abrasion takes place either in winding or in sewing,nor, following prolonged storage in bobbin form, is there any adhesivecoalescing of yarn layers.

In the same way, the above-described dip impregnation can also beeffected with a commercially available 35% strength fluoropolymerdispersion (®Hostaflon TFB X 7100 from Hoechst AG) into which. 1.5% byweight of1,3-bis-(4-methyl-3-isocyanatophenyl)-1,3-diazacyclobutane-2,4-dione(for example ®Desmodur TT from Bayer AG) has been homogeneouslyincorporated beforehand.

To test the quality of the sewing thread produced, it is subjected to atough sewing test such as that described in DE Offenlegungsschrift3,431,834.

In this test, the thread is used in sewing under the followingconditions: industrial sewing machine Pfaff 363, stitch length 1 mm,zigzag seam 8 mm width, 3,250 cycles/minute, 200 cN pre-tension, 4layers of cotton twill. The thread bonded according to the inventionpermits in this test on average over 4,000 stitches without threadbreakage, while an unbonded thread permits on average only about 300stitches.

I claim:
 1. A textile sheet coated or bonded with at least onefluoropolymer layer, the layer immediately adjoining the surface of thesheet being prepared from a formulation comprising a fluoropolymer andan adhesion-promoting constituent selected from the group consisting of1,5-diisocyanatonaphthalene, dimerized or trimerized diisocyanatoluene,and adducts of diisocyantoluene with trimethylpropane andtris-[isocyanatohexyl]biuret, the adhesive strength of the layer to thesheet being at least 10 daN/5 cm as determined by DIN 53
 530. 2. Thesheet of claim 1, wherein the sheet is comprised of synthetic fibers. 3.The sheet of claim 1, wherein the sheet is coated with additionalfluoropolymer layers.
 4. The sheet of claim 1, wherein the fluoropolymeris a fluorocopolymer containing at least 40% tetrafluoroethylene units.5. The sheet of claim 1 wherein the adhesion promoting constituent is adimerized diisocyanatotoluene.
 6. The sheet of claim 5 wherein thedimerized diisocyanatotoluene is a uretdione of diisocyanatotoluene. 7.A yarn comprising synthetic fibers or filaments coated with afluoropolymer layer, the layer being prepared from a formulationcomprising a fluoropolymer and an adhesion-promoting constituentselected from the group consisting of 1,5-diisocyanatonaphthalene,dimerized or trimerized diisocyanatoluene, and adducts ofdiisocyantoluene with trimethylpropane and tris[isocyanatohexyl]biuret,the adhesive strength of the layer to the yarn being sufficient towithstand mechanical stress when the yarn is further processed.
 8. Theyarn of claim 7, wherein the fluoropolymer is a fluorocopolymercontaining at least 40% tetrafluoroethylene units.
 9. The yarn of claim7, wherein the adhesion promoting constituent is a dimerizeddiisocyanatotoluene.
 10. The yarn of claim 9, wherein the dimerizeddiisocyanatotoluene is a uretdione of diisocyanatotoluene.
 11. The yarnof claim 7, wherein the yarn is a sewing yarn.